There is known a technique in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system and an amorphous silicon film is scanned and irradiated with the linear beam so as to crystallize the amorphous silicon film. This technique is also called excimer laser annealing (ELA).
Excimer laser annealing is advantageous in that crystallization of silicon can be conducted by scanning the silicon with a linear beam without heating a glass substrate over which an amorphous silicon film is formed to high temperature. However, there is a problem in that irradiation unevenness is generated in scanning with a linear beam when an intensity distribution of the linear beam changes. This irradiation unevenness varies a semiconductor characteristic within the substrate. When a display device with a driver and a pixel integrated (system-on-panel) is manufactured using a film having irradiation unevenness, the irradiation unevenness appears directly on the screen of the display device.
In the case of forming a linear beam by a laser irradiation apparatus, a laser beam emitted from a laser oscillator (hereinafter referred to as an “original beam”) is generally divided by a cylindrical lens array. Then, the divided laser beams pass through a cylindrical lens to be superposed, and thus a linear laser beam is formed on an irradiation surface. When this linear beam is used to process a semiconductor film, it is required that the linear beam has high homogeneity in which the energy density fluctuates within several %.
However, when this requirement is not satisfied, for example when the linear beam does not have a homogeneous energy distribution in the longitudinal direction, it is difficult to perform laser processing over the entire region in the longitudinal direction within the optimal energy range. Accordingly, it becomes impossible to process the entire surface of the substrate homogeneously, and thus irradiation unevenness occurs.
Homogeneity of a linear beam is affected by an intensity distribution of an original beam. Generally, in order to increase the homogeneity of the linear laser beam, the number of cylindrical lenses constituting a cylindrical lens array is increased so that the original beam is divided into more beams. However, with the increased number of divided beams, the interference between the divided beams becomes more outstanding, which may result in the appearance of an interference fringe on an irradiation surface. Moreover, when a cylindrical lens array is manufactured with cylindrical lenses each having a high aspect ratio between its width and length, enough intensity and lens accuracy are difficult to be achieved; therefore, it is difficult to obtain a desired beam. For this reason, actually, it is difficult to increase the number of divided beams more than a predetermined number in laser irradiation.
In order to overcome such problems, proposed is a laser annealing apparatus which eliminates influences caused by fluctuation of an intensity distribution of an original beam by devising a shape and a structure of cylindrical lenses and which realizes high homogeneity (for example, Reference 1: Japanese Published Patent Application No. H10-253916).